What security software are you running? You should have Windows Sandbox enabled for testing. https://www.howtogeek.com/399290/how-to-use-windows-10s-new-sandbox-to-safely-test-apps/ You can also boot to safe mode with networking to test.

Lets start with "Two different PCs, two different OSs" It is not really 2 diff OSs, just 2 versions of the same. PCs/desktops really is your issue, not firefox's, some chips run better that others depending on a myriad of things, hardware and desktop environments being 2. then "I always send you these reports but in about a year, the problem remains the same. Nothing has improved, I never hear back from you." That blanket statement is rubbish. You report crashes, fine, but you don't have a clue what you are talking about since you don't have a clue what your issue is. mozilla/firefox deal with tons of crashes every day. Some are probably caused by cockroaches in a heatsink. Can you say yours isn't? Their software improves daily as other complaints come in - don't take it personal you don't a reply. and "Both Firefox installations crash constantly, ie every 10 - 15 mins." There is obviously something wrong with your setups. Try using something like https://www.hirensbootcd.org/ to locate problems in your win installs which may make it easier for you to ask an intelligent question. windows has tons of ways to look into problems as well and posters here can help better than I - I use linux for a reason.